I tried it the way you described, but it
didn't worked. Because the background of the
images is always identical, a panorama tool
can't stich the images. I take one image of
the street without car and photoshoped it, so I
could assemble it seamless to a long street
without cars. Then I copied the cars from the
images on this street. This was easy, because they
fit perfekt... I didn't need to mask the cars
precisely.

Can you post one of the source images to flickr or
somewhere? I know this was taken by pointing the
camera at a fixed spot across the road, waiting
for a car to come in view than snapping the
picture, and assembling the images into a
"panorama." I'm really curious what
the individual frames looked like before
stitching.